The question is worded a little awkwardly. The lift force is experienced by the aircraft, not by the air.

Now, you know the net force on the aircraft is equal to its mass times its acceleration. What two forces does the aircraft feel? It feels a gravitational force pulling it down, and a lift force pushing it up. The sum of these two forces is equal to its mass * acceleration. Does this make sense?

Think about it this way: what's the first force felt by the aircraft? Gravity. It pulls the aircraft down. The magnitude of the force is

[tex]F_g = mg[/tex]

What's the second force felt by the aircraft? The lift. This is the force you're asked to find. Call it [itex]F_L[/itex].

What's the sum of the forces? One pulls down, one pushes up -- they are in opposition. They counteract each other. Let's call the upward force positive, and the downward force negative. The net force is: